A number of semiconductor device fabrication procedures include processes in which a material is sputtered from a target onto a workpiece such as a semiconductor wafer. Material is sputtered from the target, which is appropriately biased, by the impact of ions created in the vicinity of the target. A certain proportion of the sputtered material may be ionized by a plasma such that the resulting ions can be attracted to the wafer. The wafer is mounted on a support and is usually biased to a potential selected to attract the sputtered, ionized material. Typically, the sputtered material is composed of positive ions and the workpiece is negatively biased.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate at angles which are oblique to the surface of the substrate. As a consequence, high aspect ratio (depth to width) features such as trenches and holes on a substrate surface may not be completely filled during deposition because deposition material may build up near the top edges of the high aspect ratio feature and close off a void or cavity. For example, as illustrated in FIG. 1a, sputtered material 20 may build up near the upper edges of the high aspect ratio trenches 22 located between features 26 on a substrate 28. As the sputtered material 20 accumulates, a void or cavity 24 may become closed off within each trench 22, as illustrated in FIG. 1b. To inhibit the formation of such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In many high density plasma applications, it is preferable for the chamber to be operated at a relatively high pressure so that the frequency of collisions between the plasma ions and the deposition material atoms is increased to thereby increase the residence time that the sputtered material remains in the high density plasma zone. However, scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms typically causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. It has been found that the deposition layer can be made more uniform by reducing the distance between the target and the substrate which reduces the effect of the plasma scattering.
On the other hand, in order to increase the plasma density to increase the ionization of the sputtered atoms, it has been found desirable to increase the distance between the target and the substrate. The coil which is used to couple energy into the plasma typically encircles the space between the target and the substrate. If the target is positioned too closely to the substrate, the ionization of the sputtered material can be adversely affected. Thus, in order to accommodate the coil which is coupling RF energy into the plasma, it has often been found necessary to space the target from the substrate a certain minimum distance even though such a minimum spacing can have an adverse effect on the uniformity of the deposition.
In addition, certain chambers include a coil which is fabricated from a material which can sputtered. In such chambers, the coil acts as an additional target for sputtering. This feature allows the deposition profile to be modified and may help to improve the uniformity of deposition material on the substrate.
It has also been proposed to utilize more than one target. For example, U.S. Pat. No. 5,178,739 describes a chamber having both an end target and a cylindrical target to increase the deposition rate and improve uniformity. However, it is believed that the ionization rate of the sputtered material may not be sufficiently high for many applications.